Fluorescent lamps have a negative resistance characteristic once the gas in the lamp is ionized. This means that as current begins to increase through the lamp, the resistance of the lamp decreases. This resistance decrease causes the current further to increase, so that unless some current limiting ballast means is provided, the lamp will be destroyed. Thus, a ballast system is required which will enable the lamp to operate at a sufficiently high current for proper illumination, but will prevent the current from increasing to a level at which the lamp will destroy itself. In addition, the lamp exhibits a very high effective resistance until the gas within the lamp ionizes, at which time a much lower resistance is presented. For that reason, the fluorescent lamp requires a high starting voltage in order that the lamp may be ionized. For many years the iron core transformer ballast system, which applies power to the lamp at a frequency of 60 Hz, was the only type available, which was capable of providing a high starting voltage and of limiting the normal operating current to an appropriate level, and it was extensively used despite its several undesirable characteristics. The undesirable characteristics of the iron core transformer ballast system include low power efficiency, irritating audible buzz, high weight, the requirement for a substantial amount of iron, and a light flicker which has a tendency subliminally to make people uncomfortable.
Attempts to improve the power efficiency of fluorescent lamp ballast systems in general in the prior art have lead to the provision of solid state high frequency electronic ballast systems. High frequency is desired, because both the ballast system and the fluorescent lamps themselves are more efficient at frequencies above 400 Hz. The prior art solid state systems originally were large and complex and were only applicable to central distribution systems for controlling a number of fluorescent lamps. Recently in the prior art, however, smaller high frequency solid state ballast systems have become available which are capable of being operated in conjunction with individual fluorescent lamp fixtures. These more recent solid state ballast systems have the advantage over the prior art iron core ballasts in that they are of smaller size, less weight, need substantially less iron, produce virtually no audible noise, and have a potential for less light flicker and increased power efficiency.
There is no question but that solid state electronic ballast systems will replace all conventional iron core ballasts in the near future, particularly as the cost of electrical energy increases, and as capability and reliability of the solid state ballast systems improve.
Solid state electronic ballast systems prior to the present invention have manifested certain problems which have prevented such prior art systems from fully realizing their potential advantages. The electronic solid state ballast system of the present invention, as will be described herein embodies unique concepts and techniques which solve the problems encountered with the prior art systems, thereby advancing the state of the art for solid state electronic high frequency ballast systems.
The problem encountered with the prior art solid state ballast systems is that after the fluorescent lamp has reached its ionization state, it exhibits negative resistance characteristics as noted above. This means that its resistance varies inversely with applied power or current. This negative resistance characteristic is normally more easily controlled by iron core transformers than by solid state circuitry. This is because most of the appropriate solid state circuits are constant voltage output devices which cannot accommodate the extreme reduction in the effective resistance of the fluorescent lamp when its gas ionizes. The solid state ballast system of the present invention, however, as will be described, overcomes the problems by using a resonant inverter whose impedance is matched to the particular fluorescent lamp being operated, and which is ideal for ballast purposes. Resonant inverters are similar to constant current devices, that is, they can accommodate loads varying all the way from open circuit to a total short circuit, and this feature renders resonant inverters well suited for use in fluorescent lamp ballast systems.
A second major problem encountered in the use of solid state ballast systems in the prior art, and one that has not been adequately solved prior to the present invention, is that of power factor. Power factor is the ratio of real power to reactive voltamperes, and it is important in determining the utility transformer and power line rating. A power factor of 95% is generally considered the minimum acceptable by the power companies. Below this, larger transformers and wire sizes become necessary to deliver a given real power to the user. For that reason, it is common practice for the power companies to charge a premium to large scale power users who have poor power factors.
The prior art electronic ballast systems which incorporate inverters have dealt ineffectively with the two apparently conflicting requirements, that is, a high power factor and a minimal light flicker. Minimal light flicker is obtained only when the direct current voltage driving the inverter in the ballast system is substantially constant, that is, only when the direct current drive voltage exhibits relatively small 60 Hz ripple. The usual means for reducing the 60 Hz ripple is to filter the direct current voltage with a large filter capacitor. Unfortunately, such a large filter capacitor causes the line current to flow in short pulses, and a poorer power factor results.
The ballast system of the present invention includes a circuit which removes the conflict between obtaining both a good power factor and minimum light flicker. This circuit causes the alternating current line current to vary proportionally and in phase with the alternating current line voltage, thus providing a good power factor. Power factors of greater than 98% are typically obtained by the system of the present invention, as compared with 97% for iron core ballasts. Moreover, light flicker of no more than 2% is obtained by the system of the present invention, as compared with 35%-40% for the iron core ballasts.
The invention provides, therefore, an electronic solid state ballast system which operates to control either standard or energy saving fluorescent lamps, and which uses 25%-30% less power than the prior art iron core ballasts while providing the same visible light output. Moreover, the electronic ballast system of the invention provides a virtually flicker-free light output, a high utility line power factor, and either alternating current or direct current operation. The ballast system of the invention also provides a dimming control for the fluorescent lamps.
The ballast system of the invention supplies power to the fluorescent lamp at high frequency, which in a typical embodiment is greater than 20 KHz. This high frequency operation permits the electronic ballast system of the invention to be substantially smaller in size and more power efficient than the prior art iron core ballast. The fluorescent lamp operated by the system of the invention it itself more efficient, that is, it produces more lumens per watt, at the higher frequency. An additional benefit of the high frequency operation obtained by the system of the invention is that the time between cycles is shorter than lamp plasma relaxation time which allows the lamp to be dimmed effectively as will be described.
As explained above, prior art solid state electronic ballast systems prior to the system of the present invention were forced to trade off between 60 Hz light output flicker and an acceptable utility line power factor. Relatively little flicker could be achieved by the prior art systems, but only at the expense of poor power factor. The system of the present invention solves this problem in that substantially all flicker is removed, and yet the power factor is still maintained greater than 95%. Moreover, the system of the invention achieves a high power efficiency through the use of a switching resonant inverter output circuit, which will also be described.